1. Field of the Invention
The present invention relates to the extension of the service area in a code division multiple access (CDMA) mobile communication system and, more particularly, to an apparatus and method for extending the service area in a mobile communication system, in which the service area is defined with a normal mode or an extension mode based on a distance between the base station and the mobile station and, in case of the extension mode, a transmission clock used by the mobile station for transmitting signals is changed, thus extending the service area.
2. Discussion of Related Art
In general, all the base stations of a digital mobile communication system (digital cellular mobile communication system or personal communication system) employing CDMA mode synchronize with the CDMA system reference time. Its mobile stations also synchronize with the CDMA system reference time using time information supplied by the base stations. Here, the base station acquires the system reference time using the GPS time while the mobile station synchronizes with the base station using the time information transmitted therefrom. Accordingly, the reference time of the mobile station is delayed by the propagation delay time during which the time information is sent from the base station to the mobile station. The degree of this delay depends on the relative distance between the mobile and base station and surrounding environments.
The mobile station uses an access channel to have access the base station system located in a corresponding coverage area. The access channel is composed of access channel slots and used when the mobile is registered to the base station through the backward channel or requires a call. Here, a single access channel slot is allocated for a single access channel message transmission. This access channel message consists of four frames (20 ms) minimum to twenty-six frames maximum based on its length, and uses an appropriate value according to systems. The access channel message transmits a message frame, being synchronized with the point of time at which the slot starts, and sends the next message frame after the slot starts. Thus, the boundary of the access channel slot coincides with that of the frame.
Meanwhile, the mobile transmits an origination message using the access channel when it attempts to have access the base station system located in the coverage area. At this time, a single access channel slot is used for a single origination message transmission. The origination message includes information on the mobile (mobile device number, mode indicating whether the mobile is in the CDMA dedicated mode or dual mode, and class indicating whether the device is the PCS or cellular phone) and terminating numbers. When the mobile receives the acknowledge, for the origination message transmitted through the access channel, from the base station system within a predetermined period time, the attempt to have access the system is successively terminated. However, when the mobile does not receive the acknowledge from the system within the predetermined period of time, it retransmits the origination message and waits for the acknowledge. If the mobile did not acquire the acknowledge again, it attempts to retransmit the origination message one more time. The number of times of retransmission is predetermined (it is decided by the sum of Num_Step and Max_Req_Seq in the access parameter message sent from the base station to the mobile station) and, if the mobile does not receive the acknowledge from the system even when it performs retransmission a predetermined number of times, the attempt to access the system is finished, being failed.
Accordingly, the base station continuously monitors the access channel because it does not know when the mobile attempts the access. Here, it is inefficient to search for the access signal continuously because the mobile transmits the message using the access channel slot, and it is the most efficient for capture of the signal from the mobile to search for the access signal from the mobile within a predetermined period time during which the access signal can arrive at every frame boundary of 20 ms. The period of time during which the base station search for the access signal sent from the mobile is called xe2x80x9csearch windowxe2x80x9d. This search window is determined with regard to the propagation delay according to the service area of the base station and the basic processing delay, the service area of the base station being decided at the stage of designing and optimizing the base station. The service area of each base station can be appropriately adjusted according to surrounding environments, neighboring base stations or population density. For example, the service area is defined small (for instance, 1-3 km approximately) when the base station is located in an area where population density is higher and there are lots of neighboring buildings, while it is decided wide (for instance, 20-30 km approximately) when the base station is placed in an area where the population density is lower and there is barely building.
Meantime, the period of time during which the access signal of the mobile can arrive at the base station is calculated as below. The period of time during which the access signal can reach the base station becomes the minimum when the mobile is placed right next to the base station (when there is no aerial propagation delay) This period of time corresponds to the sum of the transmission delay of the base station, the processing delay of the mobile and the reception delay of the base station. Furthermore, the period of time during which the access signal can arrives at the base station becomes the maximum when the mobile is located at the outmost region of the service area of the base station. This period of time corresponds to the sum of the minimum period of time and double of the aerial propagation delay. Here, the aerial propagation delay is doubled because the mobile sends the signal to the base station on the basis of the time information of the base station. The actual period of time that the mobile acquired corresponds to the delay time between the base station and the mobile. Accordingly, the period of time during which the signal reaches the base station when the mobile attempts to have access it should be the sum of the delay time during which the time information is sent to the mobile from the base station and the delay time during which the signal is transmitted from the mobile to the base station. This is the reason why the aerial propagation delay is also called the round trip delay (RTD).
Meanwhile, when the mobile sends the originating message, the preamble as many as numbers decided by its operator is transmitted first, and the base station channel element starts demodulation using this preamble. Here, the maximum allowable delay in the mobile signal sent to the base station is determined according to the size of the window set in the base station. When the preamble is searched, for instance, the search is carried out from the place distant from the system time (access channel slot) by preamble_PN_offset to preamble_window_length+preamble_PN_offset in response to Half PN chip. Accordingly, the base station sets PN offset to the maximum correlation value among correlation results as many as (preamble_window_length)/4.
However, the aforementioned conventional mobile communication system cannot process a signal having a delay of two Walsh symbols or more in terms of the chip characteristic of cell site modem (CSM). That is, when the signal from the mobile arrives at the base station, being delayed more than two Walsh symbols, the base station cannot receive the signal. Here, the delay of two Walsh symbols means an area where the basic communication can be actually carried out.
Accordingly, the present invention is directed to an apparatus and method for extending the service area in a mobile communication system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an apparatus and method for extending the service area in a mobile communication system, in which the service area is defined with a normal mode or an extension mode based on a distance between the base station and the mobile station and, in case of the extension mode, a transmission clock used by the mobile station for transmitting signals is changed, thus extending the service area.
To accomplish the object of the present invention, there is provided an apparatus for extending the service area in a mobile communication system, comprising: means for generating a clock required for receiving a signal from a base station and transmitting a signal from a mobile station; means for receiving the signal transmitted from the base station; and means for variably controlling the clock generated by the clock generating means according to the signal received by the receiving means, and sending the controlled clock to means for transmitting the signal from the mobile station.
The normal mode is an operation mode in case that the distance between the base station and mobile station is included in a basic coverage area, and the extension mode is an operation mode in case that the distance between the base station and mobile station is not included therein.
To accomplish the object of the present invention, there is also provided a method for extending the service area in a mobile communication system, wherein a method of accessing a base station of a CDMA mobile communication system comprises: a first step of attempting access in the normal operation mode; a second step of performing a predetermined following operation when the access is successful, and converting the operation mode to the extension mode when the access fails; a third step of varying a transmission clock by a predetermined level after the operation mode is converted to the extension mode, and attempting the access again; and a fourth step of carrying out a predetermined following operation when the access is successful in the third step, converting the operation mode to the normal mode when the access fails after the access is attempted a predetermined number of times and returning to the initial state.
The third step comprises the substeps of: controlling the mobile signal transmitting clock to be advanced by the initial delay time; reattempting the access with the controlled transmission clock, and comparing the number of times of that access attempts that have been made so far with a predetermined number of times of access attempt when the access fails; controlling the transmission clock to be advanced by a predetermined level by steps when the number of times of the access attempts that have been made so far is less than the predetermined number of times, and then reattempting the access with the controlled transmission clock; and repeating the control of the transmission clock""s level and access attempt until the number of times of the access attempts becomes the predetermined number times when the reattempted access fails.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.